In certain communication systems, such as, in an overlay coded signal control system for railroad and mass and/or rapid transist operation, the overlay control equipment provides for the transmission and reception of signal aspect information through the track rails for train operation between headblocks. In single direction traffic territory, a transmitter connected at one end of a block section sends an encoded signal to a tuned receiver connected to the other end of the block section. In two-direction traffic territory, a transmitter and a receiver are connected at each end of the block section for providing full duplex transmission of data. The present and main concern of the subject invention involves the high power audio frequency transmitters which are commonly used in the coded signal control system. In particular, the chief interest resides in the code rate generator and carrier frequency oscillator which form two essential parts or circuits of the audio frequency transmitter. In the past, the oscillator of the code rate generator was highly sensitive to noise on the d.c. supply line and to variations of the d.c. supply voltage. The former resulted in stability problems while the latter precluded the possible generation of the desired sine wave modulating signal. It was found that the use of a voltage regulating device, such as a zener diode, in communication with the code rate generator overcame the above-noted problems; however, another problem arose when any inadvertent significant increase in the resistance appeared in series with the Zener diode. For example, a cold solder joint or connection causes an increased resistance which could result in an unsafe increase of the output signal. It has been found that the deleterious effects of a high resistance connection can be eliminated by uniquely and vitally combining the code rate oscillator with the audio frequency oscillator of the audio frequency transmitter of the coded signal control system.